Describe the preparation of potassium dichromate from iron chromite ore. What is the effect of increasing pH on a solution of potassium dichromate?
Potassium dichromate is prepared from chromite ore (FeCr₂O₄) through the following steps:
First, chromate is obtained by the fusion of chromite ore with sodium carbonate in free access of air. This produces sodium chromate.
Sodium dichromate is more soluble in water than potassium dichromate. Therefore, potassium dichromate is prepared by treating the solution of sodium dichromate with potassium chloride:
Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl
Orange crystals of potassium dichromate crystallise out from the solution.
Effect of increasing pH:
Chromates and dichromates are interconvertible in aqueous solution depending upon the pH of the solution. When pH is increased (solution becomes more basic), the dichromate ion (Cr₂O₇²⁻) converts to the chromate ion (CrO₄²⁻) according to the reaction:
Cr₂O₇²⁻ + 2OH⁻ → 2CrO₄²⁻ + H₂O
Thus, increasing pH causes the orange-coloured dichromate solution to change to yellow-coloured chromate solution.
Explanation
The answer covers both parts of the question as per NCERT textbook content. The preparation involves fusion of chromite ore with sodium carbonate, followed by conversion to sodium dichromate and then to potassium dichromate using KCl. The pH effect is explained through the equilibrium between dichromate and chromate ions - increasing pH (adding OH⁻) shifts equilibrium towards chromate formation.
Solution Steps
Step 1: Fusion of chromite ore (FeCr₂O₄) with sodium carbonate in free access of air to obtain chromate
Step 2: Conversion to sodium dichromate
Step 3: Treatment with potassium chloride to form potassium dichromate
Step 4: Orange crystals of K₂Cr₂O₇ crystallise out
Step 5: Effect of pH - increasing pH converts dichromate to chromate via reaction with OH⁻